# srand

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< c‎ | numeric‎ | random

Defined in header ``<stdlib.h>`` |  |
void srand( unsigned seed ); |  |

Seeds the pseudo-random number generator used by rand() with the value `seed`.

If `rand()` is used before any calls to `srand()`, `rand()` behaves as if it was seeded with srand(1).

Each time `rand()` is seeded with the same `seed`, it must produce the same sequence of values.

`srand()` is not guaranteed to be thread-safe.

### Parameters

seed | - | the seed value

### Return value

(none)

### Notes

Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to `rand()`, and the start of the program. It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers.

Standard practice is to use the result of a call to time(0) as the seed. However, `time()` returns a time_t value, and `time_t` is not guaranteed to be an integral type. In practice, though, every major implementation defines `time_t` to be an integral type, and this is also what POSIX requires.

### Example

Run this code

    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>

    int main(void)
    {
        srand(time(NULL)); //use current time as seed for random generator
        int random_variable = rand();
        printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable);
    }

Possible output:

    Random value on [0 2147483647]: 1373858591

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.22.2.2 The srand function (p: 252-253)

- C11 standard (ISO/IEC 9899:2011):

  - 7.22.2.2 The srand function (p: 346-347)

- C99 standard (ISO/IEC 9899:1999):

  - 7.20.2.2 The srand function (p: 312-313)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.10.2.2 The srand function

### See also

rand | generates a pseudo-random number (function)
RAND_MAX | maximum possible value generated by rand() (macro constant)
C++ documentation for srand

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